Glutamine (Gln) supplementation to enteral and parenteral nutrition has been used as a therapeutic modality to enhance mucosal integrity and function during critical illness, traumatic injury, and sepsis. Gln is a conditionally-essential amino acid that is involved in a variety of biochemical processes in the cell that include nucleic acid synthesis, nitrogen transport and balance, precursor for the tricarboxylic acid (TCA) cycle, glutathione synthesis, and protein synthesis. Gln is known to be the primary energy source for entercytes, but the molecular mechanisms by which Gln exerts its cytoprotective effects have not been determined. Gastrointestinal (GI) homeostasis is achieved through a balance between enterocyte proliferation, differentiation, and programmed cell death (apoptosis). Preliminary studies in our laboratory show that Gln starvation induces apoptosis in rat intestinal epithelial (RIE-1 ) cells. The purpose of this study is to determine the molecular mechanisms by which Gln exerts its anti-apoptotic effect. The role of Gln as a pre cursor for ATP production by the TCA cycle, and a precursor for gluthathione synthesis and their relationship to the protection/induction of apoptosis in RIE -1 cells will be investigated. These studies will provide valuable information on the molecular and biochemical mechanisms by which Gln starvation induces apoptosis in an enterocyte model. Furthermore, the application of this in vitro system to future in vivo models will further define the role of Gln in the maintenance of GI mucosal integrity and function, and may provide new means by which clinicians treat gut pathology found in nutritionally depleted patients during periods of critical illness.